1. Field of the Invention
This invention relates to a semiconductor package using an aluminum nitride substrate.
2. Description of the Related Art
Generally, plastic packages, metallic packages, and ceramic packages are used for packaging semiconductor elements. Among other packages mentioned above, ceramic packages particularly manifest high reliability in effecting airtight sealing of LSI's and enjoy prominence in moistureproofness and ability to radiate heat. The ceramic packages, therefore, are utilized in packaging a CMOS gate array, an ECL gate array, etc. which are used in arithmetic units of computers.
These ceramic packages structurally are known in such forms as DIP (dual in-line packages) and QFP (quad flat packages) using a lead frame, PGA (pin grid packages) using a lead pin, and LGA (land grid packages) using a land for input/output operations, for example. Among other ceramic packages mentioned above, the ceramic packages which use a lead frame are simple in construction and inexpensive to manufacture and, therefore, are widely used in various kinds of semiconductor elements. Particularly, QFP's find extensive utility because they can tolerate addition to the number of input/output signals and because they are of the type forming a surface package.
For the lead frames of the ceramic packages, such Fe--Ni type alloys as 42 wt%Ni--Fe and 29 wt%Ni-16 wt%Co--Fe are generally used. Since these Fe--Ni type alloys are ferromagnetic materials, however, they are at a disadvantage in entailing growth of resistance due to the skin effect either immediately after the rise time of a pulse signal or during the occurrence of a high frequency signal or suffering the magnitude of inductance to be affected by frequency. FIG. 7 illustrates the dependency on frequency of the resistance of a QFP using a lead frame made of 42 wt%Ni--Fe. FIG. 8 illustrates the dependency on frequency of the inductance of the same QFP as mentioned above.
Since the dependency of the resistance and that of the inductance on frequency invariably have adverse effects on the operating characteristics of semiconductor elements, a great need has been felt for improvements capable of eliminating these adverse effects. Particularly, recent semiconductor elements have been showing a clear trend toward increasing operating frequency (system clock frequency) for the sake of exalting their operating speed. Since the skin effect gains in prominence proportionately to the rise of frequency, the need to take a measure capable of overcoming this skin effect is being felt strongly. The skin effect offers a problem particularly to those semiconductor elements which have system clock frequencies of not less than 50 MHz. The semiconductor elements which have adopted increased clock frequencies tend to generate increased amounts of heat. To ensure stable operation of these semiconductor elements, it is likewise important that they should be adapted to tolerate the growth in the amount of heat to be generated.
Thus, for the semiconductor packages serving to accommodate semiconductor elements the clock frequencies of which are high, it is important to improve the electric characteristics thereof to the extent of precluding growth of resistance due to the skin effect and, at the same time, adopt a measure to cope effectively with the growth in the amount of heat to be generated.
Incidentally, Japanese Laid-Opened Patent Application No. HEI 3-167,850 discloses a semiconductor package using a lead frame which is composed of a core made of an Fe--Ni type alloy and a Cu layer clad on the surface of the core. The lead frame of the Fe--Ni type alloy has the surface thereof coated with the Cu clad layer because the Fe--Ni type alloy has a low electrical conductivity and because the increase of resistance due to the extreme reduction of the lead frame (extreme reduction of the terminal) in size must be made up for. The thickness of the Cu clad layer is specified to be in the range of 20 to 40% of the cross section of the core of the Fe--Ni type alloy. When the Cu clad layer is applied to the lead frame, the thickness itself of this layer increases. For the purpose of forming the lead frame in a stated thickness, therefore, it is only natural that the core of the Fe-Ni type alloy itself should be proportionately decreased in thickness. The lead frames are tending to decrease thickness for the sake of coping with the general upward trend of the number of input/output signals to be handled in semiconductor elements. If the core of the Fe--Ni type alloy itself decreases thickness, therefore, the lead frame will entail a decrease in the overall strength thereof and will suffer from a decline in packageability, for example.
Then, Japanese Laid-Opened Patent Application No. HEI 4-133,456 discloses a semiconductor device (semiconductor package) which comprises a lead frame (inner lead) made of an Fe--Ni alloy and thin-film layers of such a nonmagnetic material as Al or an Al alloy deposited one each on the opposite surfaces of the lead frame as by the vacuum deposition technique. When the thin-film layers of Al or an Al alloy are superposed on the opposite surfaces of the lead frame made of the Fe--Ni alloy as by the vacuum deposition technique, however, the following problems will ensue. This provision of the thin-film layers of A1 on the opposite surfaces of the lead frame requires first to have a lead frame set in a vacuum tank of a vacuum deposition device, vacuumize the vacuum tank, then cause vacuum deposition of Al on one of the opposite surfaces of the lead frame, open the vacuum tank and set the lead frame in the reversed state, again vacuumize the vacuum tank, and subsequently effect vacuum deposition of Al on the remaining surface of the lead frame. This complicated procedure is indispensable for the formation of the thin-film layers of Al on the opposite surfaces of the lead frame. This fact results in a notable increase in the cost of production of the lead frame. The ceramic packages of the form using a lead frame particularly enjoys inexpensiveness among other features. This prominent feature is lost, however, when the thin-film layers as of Al are formed on the opposite surfaces of the lead frame.